Hip prosthesis provided with a shaft inserted into the femur

ABSTRACT

A hip prosthesis includes a shaft which can be inserted into a hole in the femur without using cement. The proximal section of the shaft includes a base body having a form that is complementary to the shape of the hole in the femur into which the shaft is to be inserted. The hole is made using a tool of a shape corresponding to the base body. The base body has a cuneus rib raised on its dorsal or ventral surface that has a width in its center that is at least three times its height. The rear surface of the cuneus rib has rough and abrasive properties.

The part of a hip prosthesis assigned to the femur is equipped with ashaft inserted into a cavity which, after resection of the head and neckof the hip, is formed by a suitable tool in the spongy innercross-sectional area of the femur. If the shaft is inserted withoutusing cement, it is endeavored to make the shape of the cavity as far aspossible complementary to the shape of the shaft, so that the prosthesishas a secure and firm fit after insertion of the shaft. The proximalportion of the shaft coming to lie in the metaphyseal region of the bone(approximately above the lesser trochanter) is designed in such a waythat it can transmit not only vertical forces in the direction of thefemur but also forces extending transverse thereto and, in particular,medially directed forces. To ensure that the surface areas of theproximal prosthesis shaft oriented in the ventral and dorsal directionsalso take part in transmission of forces to the bone, it is known todesign their surface such that a form-fluid connection with the bonetissue can be obtained. Two possibilities are available for this,namely, on the one hand, a surface micro-roughness which is broughtabout, for example, by glass blasting, porous coating or the like, andallows the bone to infiltrate into the depressions and pores, and, onthe other hand, ribs which project from the base body of the shaft.These two possibilities can also be used together. Thus, it is known(EP-B-761 183) to arrange ribs on the dorsal and ventral faces of theproximal base body of the shaft, said ribs extending in the longitudinaldirection of the shaft, and their cross section increasing in a wedgeshape from distal to proximal. Such ribs are referred to below aswedge-shaped ribs. The cavity is produced using a rasp whose shapecorresponds to the base body of the shaft without the ribs. Uponinsertion of the shaft, the base body of the shaft forms a press fitwith the surface of the cavity. The wedge-shaped ribs cut into thespongy bone tissue as the prosthesis is being inserted. Because of theirwedge shape, they displace and compact the bone tissue. This contributesto the secure fit of the prosthesis. First, a macroscopic form fit isthus obtained between the bone and the prosthesis cross section, byvirtue of the rib. Second, a microscopic form fit is obtained after bonetissue has grown into the rough or porous surface structure. These twoeffects evidently occur independently of one another and are usedindependently of one another in the construction of a prosthesis.

Of course, the said compression of the bone material during insertion ofa wedge-shaped rib must not lead to the bone breaking. This danger isall the greater, the wider a rib, because the latter then directs agreater wedge surface toward the cortical bone and produces a greaterwedging force against the cortical bone. Known wedge-shaped ribs aretherefore made of a narrow design (EP-B-761 183, EP-B-159 462). For thesame reason, it is sought to deflect the wedging action of the ribs intothe tangential direction (DE-U-295 22 382, page 4, line 22). It is truethat wide ribs are also known in hip prostheses intended for cementlessimplantation (EP-A-10 70 490). However, these are ribs which, because oftheir shape, are unsuitable for displacement and compression of bonematerial and therefore require a shaping of the cavity such that theirvolume and their shape are taken into consideration from the outset inthe cavity. Relatively wide shaft projections broadening from distal toproximal are also known for prosthesis shafts which are intended to beimplanted using cement and in which, therefore, the associated tool isdesigned such that the artificial bone cavity is more voluminous thanthe shaft inclusive of its rib-like attachments.

The object of the invention is to make available a prosthesis shaft forcementless implantation with wedge-shaped ribs and a rough surface, saidprosthesis shaft permitting rapid and extensive adherence between theprosthesis surface and the bone tissue. The solution according to theinvention lies in the features of claim 1, namely, in brief, that thewedge-shaped rib with roughened rear face is made very wide. It is onaverage at least three times as wide, preferably four times as wide, asit is high.

Based on previous experience, one has to expect that a wide wedge-shapedrib of this kind will deploy its wedging effect mainly outwardly towardthe cortical bone and that, therefore, there will be a danger of thebone breaking. This danger would in fact arise if the rear face of therib were not rough. The roughness means that the bone substance whichcomes into direct contact with the rear face of the rib, and which isexposed to the wedging effect and relative movement, is abraded andcrushed and thus brought into a flowable state so that it can bedisplaced to the sides away from the rear face of the rib and can flowoff. This has two consequences. First, the force generated in thedirection perpendicular to the rear face of the rib is comparativelysmall, as a result of which the possibly damaging effect of the force onthe cortical bone is limited. Second, the rear face of the rib, in thefinal state of implantation, is situated in immediate proximity toundamaged bone substance in which the natural vascular system ispreserved, since the previously crushed bone substance has been removedby virtue of its flowability. This means that the process ofinfiltration of new bone substance reaches the porous rear face of therib very quickly after implantation and that, as a result, an intimateand extensive surface union is created after just a short time. It maywell be that such an effect also occurred in the previously knownwedge-shaped ribs with rough surface at their cross-sectional tip.However, since this effect was limited to a very small surface area, itwas not appreciable and it also made no positive contribution. At therib flanks, the situation is fundamentally different. In the case ofindividual ribs, a comparatively wide compression space is present inthe compression direction perpendicular to the flank surface, so thatalthough the bone substance is compressed and partially squeezed, thepressing is not as great as would be required for an appreciableabrasive effect of the roughness. The bone substance remains more orless in situ. Since the vessels therein are for the most part destroyed,it initially forms a barrier between the undamaged bone tissue and theprosthesis surface through which, after implantation, fresh bone tissuemust first have penetrated before it can infiltrate the rough surface ofthe prosthesis and bring about a form-fluid connection. In theinterstices between adjacent ribs, the situation is once again differentthan at the rear of the rib, because there the compression and tissuedestruction is particularly pronounced, without the destroyed tissuebeing able to be removed. In this area too, the connection to fresh bonetissue can therefore take place only with some delay.

The invention thus affords the advantage that, by virtue of theroughness of the large rear face of the rib, and because of theresulting closeness of undamaged bone tissue, a firm connection betweenprosthesis surface and bone surface can take place very quickly.

In the whole of the proximal portion, the rib is preferably at leastthree times as wide, preferably four times as wide, as it is high. Toensure that the abrasive action of the roughness during the relativemovement of the rib surface with respect to the pressed-on bone tissueis at all times sufficient compared to the squeezing generated by thewedging effect, the wedge angle, that is to say the angle between thesurface of the rear of the rib and the midplane extending in thelateral-medial direction (LM midplane) of the shaft, must not be greaterthan 5°, preferably not greater than 3.5°, and more preferably notgreater than 2.5°.

The width of the rib too, that is to say its dimension in the LMdirection, preferably increases from distal to proximal, and the anglebetween the lateral edge and the longitudinal direction of the shaftmust not be greater than 4°, preferably 3°. The same applies to thelateral edge.

In the shaft cross section, the rear face of the rib must extendapproximately parallel to the LM midplane. The angle between the rearface of the rib and the LM midplane is preferably not greater than 15°,the rib height increasing toward the lateral face. The medial flankexpediently extends from the rear face of the rib to form a sharp edgeand runs substantially perpendicular to the LM plane. The sameexpediently applies to the lateral edge too, although it is lessimportant there.

To ensure that the abrasive effect of the roughness is sufficient in thearea of the rear of the rib, the peak-to-valley height must lie between0.05 and 0.5 mm. The distance between adjacent peaks of the roughness isof the same order of magnitude.

The invention is explained in more detail below with reference to thedrawing which depicts an advantageous illustrative embodiment and inwhich:

FIG. 1 shows a side view,

FIG. 2 shows a view from the lateral direction, and

FIGS. 3-5 show cross sections through the prosthesis shaft atrespectively corresponding heights thereof.

The prosthesis is a straight shaft prosthesis, that is to say the shafthas a continuously straight longitudinal axis and is implanted in arectilinear direction into the femur. The shaft 1 comprises an in crosssection substantially rectangular base body with parallel dorsal andventral faces 3, 4 which taper in a wedge shape in the distal direction.They each enclose an angle of less than 2° with the lateral-medialmidplane 5. The lateral and medial boundary faces 6, 7 of the base bodyof the shaft likewise taper in a wedge shape in the distal direction.The proximal end is adjoined by the prosthesis neck, having a cone 9 forattachment of a-ball joint, and by a lateral wing 10 in the region ofthe greater trochanter.

A rasp (not shown) for forming the bone cavity intended to receive theprosthesis shaft has the same shape as the base body of the prosthesisshaft delimited by the faces 3, 4 and 6, 7, as is generally known, inorder to give the prosthesis shaft a secure and firm fit in the bonecavity after implantation.

On the dorsal and ventral faces 3, 4 of the base body of the shaft, andapproximately centrally with respect to the shaft axis 11, ribs 12 areattached which have the shape of a wedge with straight boundary faces.The size of these faces is the dorsally or ventrally oriented rear face13. Medially, the rib is delimited by a medial end face extendingapproximately perpendicular to the surface 3 or 4. The same applies tothe lateral end face 15.

With the center line 11, the end faces 14, 15 enclose, like the boundaryedges of the rear face 13, an angle of in each case approximately 2.5°.The angle which the rear faces 13 of the rib enclose with the LMmidplane of the shaft is 2°.

The rear face 13 of the rib extends approximately parallel to the LMmidplane 5 of the shaft. In the example shown, the deviation amounts toless than 10°. At the medial edge, the rib is slightly higher than atthe lateral edge, thereby increasing the macroscopic form-fit fortransmitting force from the prosthesis medially to the bone.

At least the rear face 13 of the rib 12 is provided with a rough and, ifappropriate, porous surface by means of sandblasting, plasma coating,flame spraying or the like. The roughness elevations are preferablysharp-edged so that, when the shaft is pushed into the bone, they actabrasively on the bone substance. Such a roughness can also be providedon the other faces of the prosthesis shaft in order to permit intimateconnection of the bone tissue with the prosthesis surface. As far as theinvention is concerned, what matters is simply the roughness of the rearface 13 of the rib. Whereas the base body of the shaft has been providedwith a complementary shape in the bone cavity and acquires the desiredpress fit therein without any appreciable additional deformation of thebone, there is no such complementary cavity form for the ribs 12. Whenthe shaft is introduced into the bone, the ribs displace an amount ofthe bone tissue corresponding to their volume. If the rear face weresmooth, the laminar structure of the bone would simply be compressed andcompacted, in which case the liquid content of the lamellar intersticeswould escape. Thanks to the abrasive quality of the back of the rib, thebone lamellas in the case of the invention are abraded and cut. In thisway, they can escape with the liquid interstitial content from the areabetween the rear face of the rib and the solid and undamaged bonetissues lying behind them. On the one hand, this reduces the pressingthat arises because of the wedge effect of the ribs between these andthe bone. On the other hand, the spongy bone substance located betweenthe rear face of the rib and the hard cortical bone is not totallycompressed and damaged. Instead, it remains intact, almost as far as therib surface, and can therefore contribute to rapid securing of theprosthesis by means of fresh bone tissue rapidly advancing to the shaftsurface and infiltrating the surface roughness thereof.

1. A hip prosthesis set comprising: a hip prosthesis a shaft which isconfigured to be inserted into a cavity in the femur without usingcement, a proximal portion of the shaft comprising a base body and acuneus rib assembly including a first rib projecting dorsally from thebase body relative to an implanted position and a second rib (12)projecting ventrally from the base body relative to the implantedposition, said rib assembly extending parallel to the shaft direction,increasing in cross section from a distal portion of the shaft to theproximal portion and having a rough surface, and a tool which isdesigned to shape the cavity so that it substantially corresponds to theshape of the base body, wherein the rib assembly is on average at leastthree times as wide as it is high and the roughness on its rear face issharp-edged with a peak-to-valley height range of between 0.05 and 0.5mm
 2. The hip prosthesis set as claimed in claim 1, wherein the hipprosthesis is a straight shaft prosthesis.
 3. The hip prosthesis set asclaimed in claim 1 or 2, wherein the rear face of the rib assemblyextends, in the shaft cross section, approximately parallel to thelateral-medial midplane of the shaft relative to the implanted position.4. The hip prosthesis set as claimed in claim 3, wherein the height ofthe rib assembly, at an edge delimiting it medially, is greater than atits lateral boundary.
 5. The hip prosthesis set as claimed in claim 1 or2, wherein a medial end face of the rib assembly extends approximatelyperpendicular to the surface of the base body of the shaft.
 6. The hipprosthesis set as claimed in claim 1, wherein the distance between thepeaks of the elevations forming the roughness is of the same order ofmagnitude as the peak-to-valley height.
 7. The hip prosthesis set asclaimed in claim 3, wherein a medial end face of the rib assemblyextends approximately perpendicular to the surface of the base body ofthe shaft.
 8. The hip prosthesis set as claimed in claim 4, wherein amedial end face of the rib assembly extends approximately perpendicularto the surface of the base body of the shaft.